Electron emissive cathode



May 6, 1941. D. s. GUSTIN EIAL ELECTRON EMISSIVE GATHODE Fil ed June 24, 1938 INVENTOR 61A. FREfMflA/ ATTO RN EY Patented 'May 6, 1941 ELECTRON EMISSIVE CATHODE Daniel 8. Gustin,

Freeman.

EastrangaN.J..assignorstoWestinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsyl- Application June 24, 1938, Serial No. 215,578

This invention relates to electric devices of the gaseous conduction type and more particularly to gaseous discharge devices of the high pressure mercury type and to the provision of electrodes therefore.

Devices of this type are well known in the art wherein a discharge is maintained between relatively cold electrodes. In order to support the resulting discharge an ioniza-ble medium, such as mercury or the like, is employed together with a rare gas to facilitate starting of the discharge.

- a discharge with such material shielded by a v It has been customary in devices of this type to employ what are known to the art as activated electrodes. These electrodes consist of a metal provided with a suitable coating, such as barium or strontium carbonates or similar alkaline earth compounds. which material-gives oil. a copious flow of electrons. causing ionization of the rare gas to facilitate starting of the discharge when the electrodes are supplied with electrical energy.

In the construction of high pressure mercury vapor lamps it is the usual practice to employ an envelope made of quartz or of glass with a high silica content in order that the envelope may withstand the operating temperatures of the lamp. These activated electrodes employing electron emissive material, such as above noted, have proven disadvantageousin lamps of this type owing to the tendency of the coating to vaporize during operation, which vaporized material impinges upon the inner walls of the glass envelope. This vaporized material apparently has a high affinity for the quartz or silica content of the envelope and forms a frosting or crystallization which impairs the emission of both the visible and invisible radiations.

It is accordingly an object of the present in vention to provide a high pressure gaseous discharge device wherein electrodes are employed which are composed of high electron emissive metal and which is not susceptible to vaporization during operation of the device.

Another object of the present invention is the provision of a high pressure gaseous discharge device having electrodes therein between which a discharge occurs upon the application of electrical energy which electrodes comprise a metal from which a copious now of electrons emanate to initiate a discharge, with the electron emissive metal held in place by a refractory material, the latter of which receives the impinging 10 Claims. (Cl. 176-126) would otherwise cause sputtering of the electron emissive component of the electrode.

Another object of the present invention is the provision of an electrode for a gaseous discharge device of the high pressure mercury type wherein the electrode is formed of a metal susceptible to a-copious flow of electrons to initiate refractory metal which assumes the discharge after initiation so as to protect the material having high electron emissivity from bombardment dumg operation of the device. a

Another object of the present invention is the provision of an electrode for a gaseous discharge device of the high pressure mercury type having a core of material susceptible to a copious flow of electrons whichis'surrounded by a helix of refractory metal, thus forming openings for the egress of electrons from the core and shielding the core from bombardment during operation of the device.

A further object of the present invention is the provision of an electrode having a core of a metal in the free state susceptible to a copious flow of electrons surrounded by a helix of refractory metal to form openings for the egress of electrons from the core, and for the. purpose of high pressure mercury type constructed in accordance with the present invention.

Fig. 2 is a fragmentary view in cross-section and on an enlarged scale showing one embodiment of the electrode forming the subject matter of the present invention.

Fig. 3 is a sectional view taken on the line III-III of Fig. 2.

Fig. 4 is an elevational view partly in crosssection of a modification of the electrode assembly as shown in Fig. 2.

Fig. 5 is a sectional view taken on the line V--V of Fig. 4.

Fig. 6 is an elevational view showing a part of the electrode as shown in Fig. 4.

Referring now to the drawing in detail, a gaspositive ions resulting from the discharge that eous discharge device of the high pressure cury type is shown in Fig. 1 which comprises an envelope 5 of quartz or hard glass having a high melting point so as to withstand the operating temperatures of the lamp. A pair of electrodes 6 and I are disposed therein at opposite ends of the envelope and are supported by leading-in conductors 8 and 9.

After exhaustion of the envelope, it is filled with an ionizable medium, such as mercury vapor, of just suflicient quantity as to become completely vaporized and at a pressure of one to several atmospheres during operation of the device, and in addition a small quantity of rare gas may be introduced to facilitate starting.

The electrodes 6 and 1 may be of identical construction, and by reference to Fig. 2 they comprise a core 9 of a suitable thermionically active metal, as distinguished from metallic oxides or their other metallic compounds, such as thorium or a metal of the thorium group, which gives off a copious flow of electrons when heated to a comparatively low temperature. Partially covering this core of high electron emissive metal is a material such as tungsten or other refractory metal 10, having a lower electron emissivity than that of the core at the operating temperatures of the device.

The partially covering refractory metal, as shown, is in the form of a helix ID with adjacent turns slightly spaced from each other so as to provide openings for the egress of electrons from the coreof high electron emissive metal. In addition, the refractory metal may be in the form of a cylinder in contact with the outer surface of the core and provided with openings or grooves for the emission of electrons from the high electron emissive core.

The electrode thus formed is supported by th leading-in conductor 8 sealed through the envelope 5, and the inner end thereof may be welded or otherwise affixed to the electrode, although in the preferred embodiment the leading-in conductor is shown concentric with the helixof refractory metal and in contact with the latter and thecore of high electron emissive metal 9, as can be more clearly seen inFig.-3.

Upon the initial application of electrical energy to the electrodes they are substantially cold and the resulting glow discharge therebetween is the result of the impression of a potential upon the electrodes in excess of the so-called breakdown voltage of the specific gas at the particular gas pressure employed and with the specific electrode spacings used. As a result of the glow discharge. the electrodes are subjected to ion bombardment and become incandesced thereby. Upon incandescence of the electrodes, the core of thermionically active metal, such as thorium, gives off a copious flow of electrons which initiates an arc discharge betwen the electrodes. Immediately following the initiation of the arc discharge, the refractory -metal, such as tungsten, partially covering the thorium functions as a shield protecting the material of higher electron emissivity from positive ion bombardment during operation of the device, despite the fact that the thorium naturally continues to give off electrons.

Due to its characteristics, the core material of thorium not only has a high electron emissivity at the operating temperatures of the devices, but in addition it is protected by the surrounding helix of tungsten assuming the discharge, with the result that no sputtering of the thorium occurs which would otherwise deleteriously afiect the envelope with a resulting de crease in the efficiency of both the visible and invisible radiations generated, inasmuch as such radiations would be absorbed by the envelope 5 instead of being transmitted therethrough.

Moreover, since the core material of thorium is in its free state, it has no chemical effect on silica and a quartz or other hard glass envelope may be used without frosting, even should there 10 be a slight sputtering of the thorium. However, as before stated, due to the protecting shield of tungsten assuming the discharge, sputtering is, as a matter of fact, substantially eliminated.

In higher wattage lamps a similar designed electrode must have a larger lead wire for carrying the current through the seal formed with the envelope, and unless provision is made to dissipate heat, a considerable amount is conducted into the seal which may result in cracking ,thereof early in lamp life. Accordingly, in

higher wattage lamps an electrode as above described may be employed, and in order to increase the current carrying capacity, as well as the heat dissipating characteristics of the electrode, it is only necessary to increase the amount of the refractory metal surrounding the core of therminonically active metal of thorium.

In these higher wattage lamps an additional helix 1!, such as, shown in Fig. 6, formed of tungsten is accordingly threaded upon the helix ill of tungsten, as shown in the electrode of Fig. 2. Moreover, this increasing in current carrying. capacity and heat dissipating characteristics of the electrode may be made byscrewing on the outer helix, as shown in Figs. 4., 5 and 6, during the original assembly of the electrode and prior to its insertion and sealing into the envelope of the device. Also the leading-in conductor 8 is provided with a cross-bar l3 to which is welded or otherwise amxed a looped support ll to which the thorium 9 and helices I0 and I2 of tungsten are secured.

A further advantage of an electrode constructed in the manner of the present invention resides in the fact that no treating previous to sealing of the electrode into the envelope is required and extreme precautions are unnecessary to prevent contamination from the atmosphere as is common in electrodes of the prior art employing barium or strontium carbonates or other alkaline earth compounds to prevent such contamination.

vIt thus becomes obvious to those skilled in the art that a gaseous discharge device of the high pressure mercury type is herein provided. Moreover, metallic electrodes are utilized which have a high electron emissivity to initiate the discharge and which are so constructed that the portion thereof having the high electron emissive characteristics is so protected or shielded from bombardment that sputtering of the electrode with frosting, or other deleterious effects, to the envelope is eliminated.

Although we have shown and described several specific embodiments of the present invention, we do not desire to be limited thereto as various other modifications of the same may be made without departing from the spirit and scope of the appended claims.

-We claim:

1. A high pressure discharge device comprising an enclosing envelope, an ionizable medium therein, and a pair of electrodes in said'envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes comprising a metal having a high melting and vaporization point, together with high electron-emissive properties when heated to initiate anarc discharge at low applied voltage, and a refractory metal of higher work function shielding said electron-emissive metal from positive ion bombardment to prevent sputtering of the latter duringoperationof said device. c

' 2. A high pressure dischargedevice comprising an enclosing envelope, an ionizable medium therein, and a pair of electrodes in said envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes comprising a core of metal having a high meltin and vaporization point and substantially constant high electron-emitting properties when heated by the ensuing glow discharge to emit a copious flow of electrons and initiate an arc discharge at low applied voltage, and a refractory metal of higher work function shielding said metallic core from positive ion bombardment to prevent sputtering of the latter during operation of said device.

3. A high pressure discharge device comprising an enclosing envelope, an ionizable medium therein, and a pair of electrodes in said envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes comprising a metal having a high melting and vapprization point, together with substantially constant high electron-emissive properties when heated to initiate an are discharge at low applied voltage, and a refractory metal of higher work function shielding said electron-emissive metal from positive ion bom- 3 bardment to prevent sputtering of the latter during operation and provided with openings therein for the egress of electrons from said electron-emissive metal.

4. A high pressure discharge device comprising an enclosing envelope, an ionizable medium therein, and a pair of electrodes in said envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes comprising a core of metal having a high melting and vaporization point, together with high electron-emitting properties when heated by the ensuing glow discharge to emit a copious flow of electrons and initiate an arc discharge at low applied voltage, and a refractory metal of higher work function surrounding said core and provided with openings for the egress of electrons from the latter when said electrode operates as cathode, and said refractory metal extending longitudinally of said electrode beyond said core in closer proximity to 5 the other of said electrodes for assuming the discharge and for shielding said core from positive ion bombardment when said electrode is operating as anode to prevent sputtering of the core with attendant blackening of the envelope during operation of said device.

5. A high pressure discharge device comprising an enclosing envelope provided with an ionizable medium therein, and a pair of electrodes in said envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes comprising a core of metal having a high melting and vaporization point and operative when heated to emit a copious flow of electrons and initiate a high current are discharge at low applied voltage, and a refractory metal of higher work function surrounding said core and provided with openings for the emission of electrons from the latter when said electrode is operating as cathode, and said refractory metal beingoi sufllcientm'ass to dissipate heat generated by the discharge to maintain the temperature 01' said core below the melting point thereof and extending longitudinally of said electrode in closer proximity to the other electrode than said core forassuming the discharge and for shielding said core from positive ion bombardment when said elec trode is operating as anode.--

6. A high pressure discharge device comprising an enclosing envelope provided with'an ionizable medium therein, and a pair of electrodes in said envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes comprising a core of thorium operating when heated to initiate a high current are discharge at low applied voltage, and a helix of tungsten having spaced turns wound around said core to form interstices serving for the egress of electrons from said core during the period when said electrode is operating as cathode, and said helix serving to shield said core from positive ion bombardment to prevent sputtering of the latter when said electrode is operating as anode.

7. An electrode for a high pressure discharge device operable at commercial potentials during the entirelife of said device to initiate and sustain an arc discharge without causing blackening of the envelope of said device comprising a core of thorium having a high melting and vaporization point and substantially constant electron emissive properties when heated by the ensuing glow discharge to emit a copious flow of electrons and initiate an arc discharge at low applied voltage during the entire life of said device, and a helix of tungsten surrounding said thorium core and forming interstices for the egress of electrons from said thorium core and for shielding said thorium core from positive ion bombardment to prevent sputtering thereof during operation of said device.

8. A high pressure discharge device comprising an enclosing envelope, an ionizable medium therein, and a pair of electrodes in said envelope between which an arc discharge occurs during operation of said device, at least one of said electrodes being a non-activated electrode comprising a metal having a high melting and vaporization point together with high electronemissive properties when heated to. initiate and sustain a high pressure are discharge at low applied voltage, and means to prevent sputtering of said non-activated electrode during operation of said device.

9. A high pressure discharge device comprising an enclosing envelope, a pair of electrodes in said envelope adapted to carry the discharge current, at least one of said electrodes being a non-activated electrode of high melting and vaporization point comprising a metal of the thorium group having high constant, electronemitting properties when heated, an ionizable medium in said envelope comprising a starting gas ionizable at low applied voltage to produce a glow discharge with attendant heating of said electrodes by positive ion bombardment and to cause the initiation of a high pressure are discharge at the low applied voltage, and a vaporizable material in sufficient quantity to become vaporized by the application of electrical energy to the electrodes to sustain said are discharge at the resulting high pressure of the ionizable medium.

10. A high pressure discharge device comprising an enclosing envelope, 9. pair of electrodes in said envelope adapted to carry the discharge current. at least one of said electrodes comprising a non-activated electrode of high melting and vaporization point and having high constant electron-emitting properties when heated and provided with a portion for concentrating the arc discharge thereon to protect the remainder of the electrode mass from the direct action of the arc discharge, an ionizable medium in said envelope comprising a starting gas ionizable at low applied voltage to produce a glow discharge with attendant heating or said electrodes by positive ion bombardment and to cause the initiation of a high pressure arc discharge at the low applied voltage, and a vaporizable material in suflicient quantity to become vaporized upon the application of electrical energy to the electrodes to sustain said arc discharge at the resulting high pressure of the ionizable medium.

DANIEL S. GUSTIN.

GEORGE A. FREEMAN. 

